%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 8-3-9 on page 172.
%% 
%%  Copyright (C) 2009 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass[]{ttctminimal}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{194.59192pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\def\yPh{\vphantom{A}}
\let\ol\overline
\StopShownPreambleCommands
\begin{document}
$\overline{x}+\overline{y}=\overline{z}$\\
$ \ol{x} + \ol{A} = \ol{z} $\\[5pt]
$ \ol{x\yPh} + \ol{A} = \ol{z\yPh} $
\end{document}